1. Field of the Invention
This invention relates to a semiconductor integrated circuit including MISFET's (Metal Insulator Semiconductor Field Effect Transistors) and, more particularly, to a semiconductor integrated circuit having output MISFET transistors which are subject to damage by static electricity.
2. Description of the Related Art
With the increasing speed of semiconductor integrated circuits constructed with MISFET's, fluctuations of ground or power supply line voltage during signal output at an external output terminal or an external input/output terminal (hereinafter, simply referred to as an external output terminal), increases the amount of noise in the signal.
A semiconductor integrated circuit generally has its power supply and ground lines for peripheral circuitry, and those for internal circuitry, divided by the pattern of its lines so that noise occurring at the lines for peripheral circuitry never affects the internal circuitry connected to their own separate power supply and ground lines.
A prior semiconductor integrated circuit constructed with MISFET's includes an input circuit which is connected to its own power supply and ground lines. The output side of the input circuit is connected to an internal circuit on an output side. The internal circuit is in turn connected to an output circuit having inverters, the inverters being connected to an external output terminal. The internal and output circuitry are connected to a second set of power supply and ground lines which are separated from, or electrically connected via respective impedances with, the power supply and ground lines for the input circuit. Therefore, the power supply and ground lines for the output and internal circuitry will not transmit, to the input circuit, power supply voltage fluctuations caused by the internal circuit. Consequently, such fluctuations will not reduce the voltage margin of the input circuit for detecting the input signal. Moreover, to further protect the circuit from malfunctions due to noise in the ground and power supply lines, the output transistors are typically connected to their power supply line and/or ground line through a high impedance.
Conventionally, the external output terminal of a semiconductor integrated circuit has a lower input impedance than the external input terminal. Therefore in the past, such semiconductor integrated circuits have had stronger resistance to damage from static electricity appearing on their external output terminal, than from static electricity appearing on the external input terminal. For conventional semiconductor integrated circuits constructed with MISFET's, the gate insulation film of the MISFET's forming the input circuit is subject to damage from a static charge on the external input or output terminal. Therefore protection circuits are generally provided for protecting the transistors of the input circuit. For example, an input protection circuit has been provided between the external input terminal and the input power supply and ground lines, respectively. Another protection circuit has been provided between the external input and output terminals.
In such a circuit, when a surge is induced by a static electrical charge on the external input terminal, the charge producing a higher voltage than the reverse breakdown voltage of the input MISFET's, the input protection circuit provides a path for escape of the static charge to either the input circuit's power supply line or ground line. Thus, the input protection circuit prevents the gate insulation film of input MISFET's from being damaged.
Also, if a static charge on the external input terminal reaches several kilovolts, while the voltage level of the external output terminal is at ground, the protection circuit between the two terminals carries the surge current to the external output terminal, thereby preventing the gate insulation film of the input MISFET from being damaged. Similarly, if a high voltage surge of static electricity is applied to the external output terminal while the voltage on the external input terminal is at ground level, the same protection circuit protects the input MISFET by conducting the charge to the external input terminal. However, these protection circuits will not protect the gate insulation film of an output MISFET of the output circuit, from the effects of a surge of static electricity therethrough between the external output terminal and the ground and power supply lines, as will be described below.
With the shrinkage of such semiconductor integrated circuits, MISFET's are now being formed with a LDD (Lightly Doped Drain) structure in order to avoid lowering of reliability caused by injection of hot carriers. However, output circuit MISFET's having LDD structures are more prone than their predecessors to damage from static electricity on the external output terminal. That is, when the power supply and/or ground lines are divided for the internal and peripheral circuitry, by having a high impedance in the lines connecting the output circuit MISFET's to the power supply and/or ground terminal, the MISFET cannot withstand the high drain to gate voltage which the high impedance maintains although the MISFET is conducting. This can present a problem for the shrinkage of semiconductor circuits which use MISFET's.